There a various causes of knee pain. Usually wear and tear occurs with age and/or overuse (for example in rugby players, tradespeople etc).
Softening of the cartilage beneath the knee cap (patella) results in small areas of breakdown and pain around the knee. Instead of gliding smoothly over the knee, the knee cap rubs against the thigh bone (femur) when the knee moves. The changes can range from mild to complete erosion of the cartilage.
Another very common cause of knee pain is over-pronation of the feet. The knee forms the link between the upper and lower leg and is a hinge joint, designed to flex and extend the lower leg, not to rotate it!
However, when the foot rolls inwards (over-pronation), the lower leg is forced to rotate, resulting in bad knee function. This will inevitably lead to wear and tear to the knee joint, causing long-term damage and pain.